1. Field of Invention
The present invention relates to a process using radiofrequency microwave energy to purify an air stream containing hazardous materials, particularly organic vapors, using carbonaceous adsorption followed by sweep gas microwave desorption with subsequent environmental cleanup.
2. Background
Hazardous waste material often occurs which contains organic compounds that are easily volatilized under common conditions. These sometimes naturally pollute a gas stream, like air, and sometimes result in polluted solids, like soil. In all instances release is not allowed without conversion to environmentally clean gases.
Such organic compounds, often called just organics, cover a wide variety and often contain halogen atoms, particularly chlorine, which originated from various previously employed solvents. For instance, a contaminated soil area which over the years had various liquid discharges containing organics dumped on it was tested and a wide range of concentrations of hazardous compounds detected. These are treated as a group to convert them into environmentally releasable compounds of water and carbon dioxide; however, to perform this the halide compounds are separately treated. The most typical halogen atoms end up as gaseous halide acids so they are commonly scrubbed with alkali and removed.
Adsorption of organics occurs readily upon carbonaceous materials, such as activated carbon. Thus a contaminated air stream passed through a bed of activated carbon will substantially purify it. Saturation of the bed will eventually occur so removal of the adsorbed organics is performed to allow recycling of the activated carbon. This desorption is conventionally performed by heating the bed to volatilize the organics. For instance, conventionally steam is employed for this task.
The subject invention employs microwaves for this desorption since activated carbon is a very good absorber of such microwaves. Then the desorbed volatiles, which are not necessarily in the same chemical form as they were when adsorption occurred, are collected by a sweep gas which is then treated to purify it using microwaves before release.
Quantum radiofrequency (RF) physics is based upon the phenomenon of resonant interaction with matter of electromagnetic radiation in the microwave and RF regions since every atom or molecule can absorb, and thus radiate, electromagnetic waves of various wavelengths. The rotational and vibrational frequencies of the electrons represent the most important frequency range. The electromagnetic frequency spectrum is usually divided into ultrasonic, microwave, and optical regions. The microwave region is from 300 megahertz (MHz) to 300 gigahertz (GHz) and encompasses frequencies used for much communication equipment. For instance, refer to Cook, Microwave Principles and Systems, Prentice-Hall, 1986.
Often the term microwaves or microwave energy is applied to a broad range of radiofrequency energies particularly with respect to the common heating frequencies, 915 MHz and 2450 MHz. The former is often employed in industrial heating applications while the latter is the frequency of the common household microwave oven and therefore represents a good frequency to excite water molecules. In this writing the term "microwaves" is generally employed to represent "radiofrequency energies selected from the range of about 500 to 5000 MHz", since in a practical sense this total range is employable for the subject invention.
The absorption of microwaves by the energy bands, particularly the vibrational energy levels, of atoms or molecules results in the thermal activation of the nonplasma material and the excitation of valence electrons. The nonplasma nature of these interactions is important for a separate and distinct form of heating employs plasma formed by arc conditions at a high temperature, often more than 3000.degree. F., and at much reduced pressures or vacuum conditions. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Supplementary Volume, pages 599-608, Plasma Technology. In microwave technology, as applied in the subject invention, neither condition is present and therefore no plasmas are formed.
Microwaves lower the effective activation energy required for desirable chemical reactions since they can act locally on a microscopic scale by exciting electrons of a group of specific atoms in contrast to normal global heating which raises the bulk temperature. Further this microscopic interaction is favored by polar molecules whose electrons become easily locally excited leading to high chemical activity; however, nonpolar molecules adjacent to such polar molecules are also affected but at a reduced extent. An example is the heating of polar water molecules in a common household microwave oven where the container is of nonpolar material, that is, microwave-passing, and stays relatively cool.
In this sense microwaves are often referred to as a form of catalysis when applied to chemical reaction rates. For instance, refer to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Volume 15, pages 494-517, Microwave Technology.
Related U.S. microwave patents include:
No. Inventor Year 4,144,189 Kirkbride 1979 4,968,403 Herbst et al. 1990 5,269,892 Cha 1993 5,268,343 Hopp et al. 1993
Referring to the above list, Kirkbride discloses regeneration of spent fluid cracking catalysts by heating with microwaves to a range of 700-900.degree. F. to remove coke; however, preheating by conventional means is suggested before usage of microwaves. The subject invention operates with much lower temperatures by microwave catalysis not just microwave heating.
Herbst et al. discloses an improvement in the regeneration of cracking catalysts by selective use of microwave heating. High temperatures in the 650-750.degree. C. range are employed. The subject invention employs microwave catalysis not just microwave heating.
Cha discloses char-gas oxide reactions, such as NO.sub.x decomposition, catalyzed by microwaves, but does not decompose general hazardous matter, like organics. Yet this shows that if any NO.sub.x was present, it is made environmentally safe.
Hopp et al. disclose a conventional reactivation process for activated charcoal catalyst used with the preparation of R-227 refrigerant by heating to the 450-900.degree. C. range. No microwaves are employed. The subject invention operates with much lower temperatures by microwave catalysis.